Secretory vesicles are transported in a highly polarized fashion to sites of cell surface growth in the budding yeast Saccharomycs cerevisiae. Polarized vesicle transport relies on the actin-based cytoskeleton, Myo2p a type V myosin and the activation of the rab GTPase Sec4p by its exchange protein Sec2p. Both Sec4p and Sec2p localize to exocytic sites through their association with secretory vesicles. Mutations in Sec2p that interfere with vesicle association, block protein secretion and result in the accumulation of vesicles randomly distributed throughout the cell. Sec2p directly interacts with Ypt32p, arab that controls Golgi function, and this interaction facilitates the association of Sec2p with vesicles. However, recent studies indicate that an additional factor must also be involved. We will use several approaches to identify this factor and to test several predictions regarding its interactions, localization and function. We will test the possibility that this factor, alone or in combination with Ypt32p, regulates the exchange activity of Sec2p. We will pursue the possibility that Sec2p interacts with the Arf GTPase, potentially linking vesicle formation to vesicle transport. We will pursue structural studies of Sec2p, alone and in combination with Sec4p, Ypt32p and the novel binding partner. Finally, we will unravel the interconnections between Sec2p, Sec4p and Ypt32p with the Myo2p myosin motor.